Re: [PATCH v2 4/6] mm: drop VMA lock before waiting for migration

[Peter Xu <peterx@redhat.com>]

On Fri, Jun 09, 2023 at 06:29:43PM -0700, Suren Baghdasaryan wrote:
> On Fri, Jun 9, 2023 at 3:30 PM Suren Baghdasaryan <surenb@google.com> wrote:
> >
> > On Fri, Jun 9, 2023 at 1:42 PM Peter Xu <peterx@redhat.com> wrote:
> > >
> > > On Thu, Jun 08, 2023 at 05:51:56PM -0700, Suren Baghdasaryan wrote:
> > > > migration_entry_wait does not need VMA lock, therefore it can be dropped
> > > > before waiting. Introduce VM_FAULT_VMA_UNLOCKED to indicate that VMA
> > > > lock was dropped while in handle_mm_fault().
> > > > Note that once VMA lock is dropped, the VMA reference can't be used as
> > > > there are no guarantees it was not freed.
> > >
> > > Then vma lock behaves differently from mmap read lock, am I right?  Can we
> > > still make them match on behaviors, or there's reason not to do so?
> >
> > I think we could match their behavior by also dropping mmap_lock here
> > when fault is handled under mmap_lock (!(fault->flags &
> > FAULT_FLAG_VMA_LOCK)).
> > I missed the fact that VM_FAULT_COMPLETED can be used to skip dropping
> > mmap_lock in do_page_fault(), so indeed, I might be able to use
> > VM_FAULT_COMPLETED to skip vma_end_read(vma) for per-vma locks as well
> > instead of introducing FAULT_FLAG_VMA_LOCK. I think that was your idea
> > of reusing existing flags?
> Sorry, I meant VM_FAULT_VMA_UNLOCKED, not FAULT_FLAG_VMA_LOCK in the
> above reply.
> 
> I took a closer look into using VM_FAULT_COMPLETED instead of
> VM_FAULT_VMA_UNLOCKED but when we fall back from per-vma lock to
> mmap_lock we need to retry with an indication that the per-vma lock
> was dropped. Returning (VM_FAULT_RETRY | VM_FAULT_COMPLETE) to
> indicate such state seems strange to me ("retry" and "complete" seem

Not relevant to this migration patch, but for the whole idea I was thinking
whether it should just work if we simply:

        fault = handle_mm_fault(vma, address, flags | FAULT_FLAG_VMA_LOCK, regs);
-       vma_end_read(vma);
+       if (!(fault & (VM_FAULT_RETRY | VM_FAULT_COMPLETED)))
+               vma_end_read(vma);

?

GUP may need more caution on NOWAIT, but vma lock is only in fault paths so
IIUC it's fine?

-- 
Peter Xu